v2: server status

[emersion]

• Plumb context
• Command pipelining (imapserver: split command handling #554)
• Expose helpers for FETCH and SEARCH
• Expose helper for making a NumSet static
• Add support for CREATE, DELETE, RENAME, PERMANENTFLAGS unilateral updates (maybe more?)
• Add Server.Shutdown (imapserver: add Server.Shutdown #665)
• Add PREAUTH greeting
• Add support for SEARCHRES (imapserver: implement SEARCHRES #549)
• Canonicalize message flags and mailbox attrs
• Add proper NAMESPACE support to imapmemserver (imapmemserver: add namespaces #509)
• Decode header fields and body in SEARCH
• Use literal8 in response to FETCH BINARY[]
• Consider allowing users to dynamically enable/disable caps
• Send FLAGS updates in imapmemserver (imapmemserver: send FLAGS updates #497)
• Add more extensions
  • LITERAL+
  • METADATA
  • QUOTA
  • SORT
  • THREAD